Ways to measure body composition

Every method estimates something different and calls the answer "body fat percentage". Knowing what each one actually measures explains why they disagree.

For a precise snapshot
  • DEXA
  • Air-displacement (BodPod)
  • Hydrostatic weighing
For tracking change
  • Photo scans
  • Tape measurements
  • Smart scales
  • Photographs in fixed conditions
Two different jobs. One-off precision and repeated tracking are not the same requirement.

DEXA

An X-ray scan that distinguishes bone, lean tissue and fat. It is the practical reference standard in consumer terms, and it is the closest most people can get to a real measurement.

Caveats: hydration and recent food affect it, machines differ, and repeated scans on different machines are not comparable. It also costs money and requires an appointment, so it is a snapshot rather than a tracking tool.

Bioimpedance — smart scales and handheld devices

Passes a small current through the body and infers composition from resistance. Convenient, cheap and extremely sensitive to hydration, time of day, recent exercise and food.

Absolute numbers are unreliable. As a daily trend under identical conditions it has some value, mostly as a smoothed weight.

Photo scans

Estimate circumferences from images with your height as scale, then derive a body-fat figure. Circumferences land within a few centimetres in good conditions; the derived percentage carries more error.

Their strength is repeatability and cost: same method, same conditions, every month, for nothing.

Tape measurements

Unglamorous and genuinely good. A waist measurement at a consistent landmark is one of the most useful numbers available, and it correlates with health risk better than body-fat percentage does.

What to actually do

Pick one method and keep it identical. The change is the signal; the absolute number is mostly noise you are comparing against nothing.

If you want one precise reading — a starting point, or curiosity — get a DEXA. For the months in between, a photo scan and a tape measure will tell you everything you need about direction.

Questions

How accurate is DEXA for body fat?

It is the practical reference standard for consumers, though hydration, recent meals and differences between machines all introduce error. It is best used as an occasional snapshot.

Are smart scales accurate for body fat?

Bioimpedance is highly sensitive to hydration and timing. Treat the absolute number sceptically and the smoothed trend as mildly useful.

Which method should I use monthly?

A photo scan and a tape measurement, both under identical conditions. Consistency of method matters more than the method itself.

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